It is well known that an image forming apparatus may include an image formation unit, which may form an image by transferring a plurality of colors of developers, e.g., color toners, and an endless belt, which may be rotatably disposed opposite to the image formation unit. In the image forming apparatus, it has been proposed that the image is formed on a recording medium transported by the endless belt. Alternatively, it has been also proposed that the image may be formed on a recording medium by transferring the developers once onto the endless belt and then transferred from the endless belt onto a recording medium.
It has been proposed that, in the image forming apparatus, color shifts are corrected by detecting registration marks, which the image formation device forms on the endless belt, based on irradiated light reflected from the endless belt. In this case, it is necessary to place a mark sensor that detects the registration marks based on the reflected light in position with respect to the endless belt. As a measure, rollers for extending the endless belt therebetween and registration mark sensors are disposed on a side plate for fixing components thereto.